1. Field of the Invention
The present invention relates to a pickup device for recording information to and reproducing the same from a magneto-optical recording medium, and also relates to a photo detecting unit in use with the pickup device.
2. Description of the Prior Art
A conventional magneto-optical recording/reproducing system detects a magneto-optical signal by using a magneto-optical light splitting element by utilizing a birefringence, such as a 3-beam Wollaston prism, in an optical system with collimiator, viz., in parallel light beams.
FIG. 1 is a diagram showing an arrangement of a conventional magneto-optical recording/reproducing pickup device. A light beam emitted by a light source 1, such as a semiconductor laser device, is converted, by a diffraction grating 2, into at least three spot light beams which will be used for generating a tracking control signal. These light beams are collimated by a collimating lens 3, usually consisting of two lenses joined together. The collimated light beams are converged on a-magneto-optical disk 6 by an objective lens 5. To reproduce information, the plane of polarization is turned in accordance with an inverted magnetization pattern corresponding to information written into a vertically magnetized film, in a recording track.
The light beams reflected by the magneto-optical disk 6 are rendered parallel by the objective lens 5 and returned to a beam splitter 4. The light beams are reflected by the beam splitter 4 toward a Wollaston prism 7. The Wollaston prism 7 separates the received light beams into an S polarized light component, a P polarized light component, and a light component as the combination of the S and P polarized light components. These polarized light components are incident on a photo detecting unit 12 through a route of a reflecting mirror 8, a converging lens 9, a concave lens 10, and a cylindrical lens 11 for obtaining a focusing error signal. The direction of the magnetization of a readout signal surface is determined by comparing the intensities of the P and S polarized light components. A focusing error signal is obtained, by the astigmatic method, from the light component as the combination of the S and P polarized light components. A tracking error signal is obtained by comparing the intensities of both sub-beams of the three beams derived from the diffraction grating. Thus, the control signals are generated for controlling the focusing and tracking directions.
Another conventional optical pickup device in use with a magneto-optical recording/reproducing apparatus containing the optical system with collimator is disclosed in Unexamined Japanese Patent Publication (Kokai) Sho-63-127436. In the publication, the parallel beams emanating from the collimating lens are optical-axis transformed (reflected) by a polarizing beam splitter. The reflecting light beams are focused on the magneto-optical disk through an objective lens. The reflecting light beams from the magneto-optical disk are converted into parallel light beams by the objective lens. The light beams, after passing through the polarizing beam splitter, are separated, by an analyzer, into an S polarized light component, a P polarized light component, and a light component as the combination of the S and P polarized light components. Finally, a magneto-optical signal and other control signals for controlling the focusing and tracking directions are formed.
The conventional optical pickup devices including the optical system with collimator is employed in order to suppress a variation of the splitting characteristics of the polarizing beam splitter 4 and the Wollaston prism 7, and to reduce a degree of deterioration of the magneto-optical signals. The objective lenses used in parallel light beams can be more easily designed and manufactured than those used in divergent light beams. For this reason, the collimating lens 3, usually consisting of two spherical glass-joined lenses, for collimating the divergent light beams, and the converging lens 9 for converging the parallel light beams are indispensable for the conventional pickup devices. Use of those lenses brings about complexity of the construction, increase of the number of the indispensable parts, and increase of the size of the optical pickup device.
To solve the problems, there is proposed an optical pickup device in use with the magneto-optical recording/reproducing apparatus, which is designed on the basis of an optical system without collimator, as shown in FIG. 2 (xe2x80x9cO plus Exe2x80x9d, No. 163, 1993, June, pp94 to 95). In the optical pickup device, divergent light beams emitted from a light source 1 pass through a convex lens 13 where a degree of the divergence of the divergent light beams is reduced. The light beams as left divergent are incident, as an S polarized light, on a plate polarizing beam splitter 14. The divergent light beams reflected by the plate polarizing beam splitter 14 are converged on the recording surface of the magneto-optical disk 6, through an objective lens 15. The reflecting light beams are converted, the objective lens 15, into the convergent light beams which in turn enter the plate polarizing beam splitter 14. In the plate polarizing beam splitter 14, the polarizing film allows part of the S polarized light beams and most of the P polarized light beams to pass therethrough. A half wave plate 17, located on the rear side of the plate polarizing beam splitter 14, turns the direction of polarization by 45xc2x0 of the light beam. Thereafter, a plate analyzer 18 splits the light beam into an S polarized light beam, a P polarized light beam, and a light beam as the combination of the S and P polarized light beams. These light beams are converted into electrical signals by a photo detecting unit 19.
In the pickup device shown in FIG. 2, to obtain exact information, it is necessary to accurately adjust the angles of the plate polarizing beam splitter 14 and the plate analyzer 18. This makes the assembling work difficult. Further, a accurate control of the thickness of the plate analyzer 18 is required. Accordingly, the manufacturing work is difficult. The half wave plate 17 is provided for turning the plane of polarization by 45xc2x0 and for disposing the plate analyzer 18 on a plane without rotating along the optical axis. This half wave plate 17 is expensive. Provision of the half wave plate 17 runs counter to the cost reduction.
Additional pickup devices based on the optical system without collimator are disclosed in Unexamined Japanese Patent Publication (Kokai) Hei-5-142419, Hei-5-142420, and Hei-5-142421. A Wollaston prism 21 as illustrated in FIGS. 3A and 3B is used. The optical system of the optical pickup device is as shown in FIG. 3C. The Wollaston prism 21 as a multifunctional Wollaston prism includes a polarizing beam splitting 21c. The polarizing beam splitting film 21c directs an incident light beam 24, which is emitted from a light source 1, toward the objective lens 15, and allows a reflecting light beam 25, which comes in through the objective lens 15, to pass therethrough. (The polarizing beam splitting film 21c is a multilayer film formed by alternately layering a plural number of dielectric thin films of different refractive indices, and is formed on the incident surface of the Wollaston prism 21.) The Wollaston prism 21 consists of a first prism 21a and a second prism 21b, both being made of crystalline and joined together along their long faces. A plane including the optical axis of the reflecting light beam 25 coming in through the objective lens 15 (the same thing is correspondingly applied to the optical axis of the incident light beam 24 emitted from the light source 1) and the optic axis 21d of the first prism 21a, is at an angle, not a right angle, to a plane including that optical axis and the optic axis 21e of the second prism 21b. The Wollaston prism 21 thus constructed is disposed slanted with respect to the optical axis in the optical path of the reflecting light beams as non-parallel light beams, whereby an astigmatism is caused.
The multifunctional Wollaston prism 21 splits the reflecting light beam 25 into P polarized light beams b, S polarized light beams c, and the light beams a as the combination of the S and P polarized light beams (FIGS. 3B). These light beams a, b, and c are received by photo detecting elements 16a, 16b, and 16c, respectively (FIG. 3C). A signal processor 16d compares the intensities of the light beams b and c, thereby reading information contained in the reflecting light beams. The photo detecting element 16a containing a 4-division photo diode is capable of producing a focusing error signal as will be described later.
The multifunctional Wollaston prism 21, which is slanted to the optical axis, causes an astigmatism, and hence substitutes for the combination of the polarizing beam splitter and the cylindrical lens. Use of this prism contributes to reduction of the number of the required parts. In the Wollaston prism 21, which is constructed such that the optic axis 21d of the first prism 21a is oriented at a right angle to the optical axis of the light beam passing therethrough, the image by the light beam emitted from the prism is not blurred.
In the construction using the multifunctional Wollaston prism 21 as shown in FIG. 3C, the pickup device produces an insufficient amount of output power to a recording medium. The axially positioning adjustment is essential to the photo detecting elements 16a, 16b, and 16c. This adjustment is laborious and difficult.
In writing information to and reading the same from a recording medium by the optical pickup device as described above, the objective lens must be exactly positioned in both the focusing direction and the tracking direction. In the magneto-optical disk as the recording medium, a Kerr rotation of the plane of polarization is read in the form of a magneto-optical signal. The magneto-optical signal is weaker than a pit signal for the compact disk.
The size reduction of the magneto-optical disk system is a recent trend in this field of the products. The pickup device in use with the magneto-optical disk system is also under a constant pressure of size reduction. In this circumstance, a unique optical pickup device has been proposed (Unexamined Japanese Patent Publication (Kokai) Hei-4-157647). In the pickup device, the combination of a diffraction grating and a 3-beam Wollaston prism, as already described, is used so as to allow a single photo detecting element to receive the reflecting light beams and to pick up thereof information recorded in the magneto-optical disk (in the form of magneto-optical signals), a focusing error signal indicative of a positional deviation (defocusing quantity) in the focus direction, and a tracking error signal indicative of a positional deviation in the track direction.
In this pickup device, three split light beams, the light beam of the 0-th order of diffraction, and the light beams of xc2x11st order of diffraction are incident on the recording layer of the magneto-optical disk. The reflecting light beams R0, R1, and R2 from the recording layer are applied to the 3-beam Wollaston prism 101 as shown in FIG. 4. The 3-beam Wollaston prism 101 further splits each of these reflecting light beams into three reflecting light beams in a direction perpendicular to the separation of the diffraction grating. Totally nine reflecting light beams R0, R1, R2, R10, R11, R12, R20, R21, and R22 are produced. Of those nine reflecting light beams, five light beams R0, R1, R2, R10, and R20 are detected by a single photo detecting unit 102 (FIG. 5). The result of the detecting is used for generating a focusing error signal, a tracking error signal, and a magneto-optical signal.
The conventional photo detecting unit 102 includes detecting elements 103, 106 and 107 for detecting the light beams R0, R1, and R2 corresponding to those of the 0th order and xc2x11st order of diffraction, which are split by the diffraction grating, and detecting faces 104 and 105 for detecting the light beams R10 and R20, which are split by the 3-beam Wollaston prism 101. The photo-detecting elements 103, 106 and 107 for the light beams R0, R1, and R2 are disposed at a right angle to the photo-detecting elements 104 and 105 for the light beams R10 and R20.
As seen, the reflecting light beams R11, R12, R21, and R22, located at four corners, are not used in the conventional pickup device. The ratio of the quantities of the 0th order to xc2x11st order of diffraction, caused by the grating, is set at a relatively small value within a range from 4 to 8, in order to increase the amplitude of the tracking error signal. The 3-beam Wollaston prism of which the basic split-light quantity ratio by the 3-beam Wollaston prism, i.e., ordinary ray:ray as the combination of ordinary ray and extraordinary ray:extraordinary ray, is 25:50:25, is frequently used. The quantity of each of the ordinary and extraordinary rays is the half of that of the combination of the ordinary and extraordinary rays. The magneto-optical signal (ordinary ray intensityxe2x80x94extraordinary ray intensity) is relatively weak.
In the above-mentioned optical pickup device, the reflecting light beams R11, R12, R21, and R22, located at four corners, are not used. Because of this, the tracking error signal is weaker than those by the light beams including those at the four corners. Since the ratio of the quantities of the light of the 0-th order of diffraction to the light of the xc2x11st order of diffraction, caused by the grating, is not large, the resultant focusing error signal and the magneto-optical signal are not large in amplitude. With regard to the basic split-light quantity ratio by the 3-beam Wollaston prism, the beam intensity ratio in the central part is large, while that on both sides is the half of that in the central part. The resultant magneto-optical signal is not large in amplitude.
A cubic beam splitter and a cylindrical lens are usually used in the conventional optical pickup device. The direction of the beam splitting by the grating is at a right angle to that of the beam splitting by the 3-beam Wollaston prism. Accordingly, the photo detecting unit is constructed such that the photo detecting elements for the tracking error signal are arrayed at right angles to the photo detecting elements for the magneto-optical signal.
Use of the cubic beam splitter and a cylindrical lens inevitably increases the number of components and the size of the optical pickup device.
To cope with this, there is proposed an optical pickup device which uses a plate beam splitter with an astigmatism causing function (for generating a focusing error signal) for size reduction purposes (xe2x80x9cO plus Exe2x80x9d, No. 163, pp93 and 94). A diffraction grating is not used in this optical pickup system. The conventional photo detecting unit of the type in which the photo detecting elements for the tracking error signal is disposed at a right angle to the photo detecting elements for the magneto-optical signal, is improperly operable when it is applied to the optical pickup system using the diffraction grating and the plate beam splitter. If applied, it fails to produce desired signals.
Accordingly, a first object of the present invention is to provide a magneto-optical recording/reproducing pickup device which includes an optical system without collimator, and has various advantages, such as a decreased number of necessary parts, a simple construction, the requirements for the parts layout accuracy being not so high when comparing with optical pickup device using the plate analyzer, a simple assembling work, and use of a Wollaston prism, easily available, being allowed.
A second object of the present invention is to provide a magneto-optical recording/reproducing pickup device which is small in size and low in cost with a decreased number of required parts by unification of the Wollaston prism, beam splitter, and an astigmatic generator element. Furthermore, the pickup device produces an increased output power to a recording medium, and allows an easy positioning of the photo detecting element on the optical axis, and an easy designing of the objective lens by use of a positive singlet.
A third object of the present invention is to provide a photo detecting unit which produces tracking signal having large amplitude.
A fourth objective of the present invention is to provide an optical pickup device using the photo detecting element of the third object.
A fifth object of the present invention is to provide a photo detecting unit which uses a plate beam splitter with an astigmatism causing function, and is well adaptable for an optical system in which the direction of the beam splitting by the grating is at an angle, not a right angle, to that of the beam splitting by the 3-beam Wollaston prism.
A sixth objection of the present invention is to provide an optical pickup device using the photo detecting unit of the fifth object.
According to first aspect of the invention, the magneto-optical recording/reproducing pickup device comprises: a light source for generating light beams; a diffraction grating for splitting the light beam emitted from the light source into at least three light beams; an objective lens for converging the light beams emitted from the light source on a magneto-optical recording medium, and receiving the reflecting light beams from the recording medium, the magnification of the objective lens being xe2x88x926.0 to xe2x88x9212.0 when an object point lies at the signal surface of the magneto-optical recording medium; a beam splitter for separating the light beams coming from the light source and incident on the objective lens from the light beams coming through the objective lens; a Wollaston prism for splitting the reflecting light beams coming from the magneto-optical recording medium through the beam splitter; and a photo detecting unit including a plural number of photo detecting elements for detecting the light beams emanating from the Wollaston prism. In the first magneto-optical recording/reproducing pickup device, a positive singlet for reducing a degree of divergence of the diverging light beams from the light source may be provided between the light source and the objective lens.
In the first magneto-optical recording/reproducing pickup device, an optical system without collimator in which a degree of convergence of the light beams incident on the beam splitter and the Wollaston prism is reduced can be constructed when the objective lens having the magnification within the above range of figures is used. The optical system of the optical pickup device may be constructed as an optical system without collimator by using the objective lens of that magnification, not collimating the light beams emitted from the light source. With this optical system, the optical pickup device succeeds in suppressing a variation of the splitting performances by the beam splitter and the Wollaston prism, and reducing a degree of deterioration of the magneto-optical signal. Further, the maximum value of the wavefront aberration measured on condition that the objective lens is moved in the tracking and the focusing direction becomes smaller, the larger a value of the magnification xcex2 is, as shown in FIG. 29. When the magnification is xe2x88x926 or greater, it is easy to form an optical system which satisfies the criterion value 0.07 xcex of Marechal. A distance from a light source to a converging point on the disc, viz., the entire length of the optical path ((L1+L2) in FIG. 6B), becomes longer, the larger the magnification xcex2 is. If a distance L1 from the converging point to the principal point is 3 mm or a bit longer, the total length of the optical path is approximately 40 mm when the magnification is xe2x88x9212. If the magnification exceeds this value, the total length of the optical path becomes large.
For disposing the Wollaston prism, the incident plane thereof is merely set perpendicular to the optical axis of the incident light beams. Moreover, the Wollaston prism may be disposed on a plane, and there is no need of using a polarization angle turning means, such as a half-wave plate. With provision-of the positive singlet between the light source and the objective lens, the distance between the light source and the objective lens is reduced. This leads to the size and cost reduction of the optical pickup device, and an efficient use of the light beam emitted from the light source. For the aberration correction, the positive singlet may be used in addition to the objective lens. This indicates an easy aberration correction of the optical system of the pickup device.
According to the second aspect of the invention, the magneto-optical recording/reproducing pickup device comprises: a light source for generating light beams; a diffraction grating for splitting the light beam emitted from the light source into at least three light beams; an objective lens for converging the light beams emitted from the light source on a magneto-optical recording medium, and receiving the reflecting light beams from the recording medium, the magnification of the objective lens being xe2x88x926.0 to xe2x88x9212.0 when an object point lies at the signal surface of the magneto-optical recording medium; a Wollaston prism for causing an astigmatism including a polarizing light splitting film for separating the light beams coming from the light source and incident on the objective lens from the light beams coming through the objective lens, the Wollaston prism being composed of first and second crystalline prisms which are joined, a plane including the optical axis of the reflecting light beam coming in through the objective lens and the optic axis of the first prism, is at an angle, not a right angle, to a plane including that optical axis and the optic axis of the second prism, the Wollaston prism is disposed in the optical path of the reflecting light beams as light beams being convergent and not parallel in a state that the incident plane thereof is slanted with respect to the optical axis, whereby causing an astigmatism, a positive single provided between the light source and the objective lens, the total magnification of the optical system including the objective lens and the positive singlet being xe2x88x920.3 to xe2x88x926.0; and a photo detecting unit including a plural number of photo detecting elements for detecting the light beams emanating from the Wollaston prism.
In the second magneto-optical recording/reproducing pickup device, the diffraction grating and the positive singlet may be formed in a single construction.
In the second magneto-optical recording/reproducing pickup device, a collimatorless optical system in which a degree of convergence of the light beams incident on the Wollaston prism is reduced can be constructed when the objective lens having the magnification within the above range of figures is used. A degree of divergence of the divergent light beams emitted from the light source is reduced by the positive singlet, the optical path length is reduced, and an output power of the optical pickup device to the object is increased. The optical system of the optical pickup device may be constructed as an optical system without collimator by using the objective lens of that magnification. With this optical system, the optical pickup device succeeds in suppressing a variation of the splitting performances by the beam splitter and the Wollaston prism, and reducing a degree of deterioration of the magneto-optical signal. The Wollaston prism is disposed in the optical path of the reflecting light beams in a state that the incident plane thereof is slanted with respect to the optical axis, whereby causing an astigmatism. For the aberration correction, the positive singlet may be used in addition to the objective lens. This indicates an easy aberration correction of the optical system of the pickup device. The light beams may be focused on the photo detecting unit by moving the positive singlet along the optical axis. When the diffraction grating and the positive singlet may be formed in a single construction, the turning of the diffraction grating or the positioning of the positive singlet along the optical axis may be adjusted by a single operation means. Further, the maximum value of the wavefront aberration measured on condition that the objective lens is moved in the tracking and the focusing direction becomes smaller, the larger a value of the total magnification xcex2xe2x80x2 is, as shown in FIG. 30. When the magnification is selected to be xe2x88x923 or greater, it is easy to form an optical system which satisfies the criterion value 0.07 xcex of Marechal. The main beam power to the disk becomes smaller, the larger the total magnification xcex2xe2x80x2 is. The reason for this is that when the total magnification is large, the quantity of light beams entering the optical system from a light source is reduced. If the total magnification is xe2x88x926 or smaller, the main beam power to the disk is 0.5 mW (the criterion value of MD) or larger.
To achieve the third object, there is provided a photo detecting unit for receiving the reflecting light beams formed in a manner that a light beam is split into at least three light beams of the 0-th order and the xc2x11st order of diffraction by a diffraction grating, the split light beams are incident on a recording medium, the light beams reflected on the recording medium are each split into at least three light beams by a Wollaston prism, the photo detecting unit comprising: a first photo detecting element for receiving the light beams of the +1st order of diffraction; and a second photo detecting element for receiving the light beams of the xe2x88x921st order of diffraction.
According to the fourth aspect of the invention, the magneto-optical recording/reproducing pickup device comprises: a light source for generating light beams; a diffraction grating for splitting the light beam emitted from the light source into at least three light beams of the 0-th order and the xc2x11t order of diffraction; an objective lens for converging the light beams split by the diffraction grating on a recording medium, and receiving the reflecting light beams from the recording medium; a Wollaston prism for splitting the reflecting light beams coming in through the objective lens into at least three light beams, a photo detecting unit for receiving at least nine reflecting light beams from the Wollaston prism, the photo detecting unit including a first photo detecting element for receiving at least three light beams split by the Wollaston prism of the +1st order of diffraction, a second photo detecting element for receiving at least three light beams split by the Wollaston prism of the xe2x88x921st order of diffraction, a third photo detecting elements, consisting of a plural number of photo detecting elements, for receiving in divided form the central light beam of the three light beams split by the Wollaston prism of the 0-th order of diffraction, and a pair of fourth photo detecting elements for receiving respectively the reflecting light beams located on both sides of the central reflecting beam of the three light beams split by the Wollaston prism of the 0-th order; and lens drive means for driving the objective lens for positioning adjustment.
In the fourth optical pickup device of the present invention, a tracking error signal, for example, may be formed by calculating the difference between the output signals of the first and second photo detecting elements of the photo detecting unit. Since at least three reflecting light beams together are incident on the first and second photo detecting elements, the tracking error signal generated is large in amplitude. A focusing error signal, for example, may be formed by the output signals of the divided photo detecting elements of the third photo detecting element. A magneto-optical signal, for example, may be formed by calculating the difference between the paired fourth photo detecting elements.
Furthermore, when the ratio of the quantities of the 0th-order light to the xc2x11st order of light is set at 8.5 or larger, the amplitude of the focusing error signal and magneto-optical signal may be increased. However, if it exceeds 15, the amplitude of the tracking error signal is too small. Therefore, a preferable ratio of the quantities of the 0th-order light to the xc2x11st order of light is within the range of 8.5 to 15. At the ratio within this range, the magneto-optical signal and the focusing error signal may be increased in amplitude in a state that the tracking error signal is not too small in amplitude.
Moreover, the Wollaston prism for splitting the received light beam into ordinary ray, extraordinary ray, and the light beam as the combination of the ordinary ray and extraordinary ray, is constructed such that the ratio of the quantities of the ordinary ray and the extraordinary ray to the whole light quantity is within 30 to 45%. When using such a Wollaston prism, the produced magneto-optical signal is large. When the photo detecting unit of the invention is further used, the tracking error signal is also large. If the ratio of the quantities of the ordinary ray and the extraordinary ray to the whole light quantity exceeds 45%, the focusing error signal is too weak.
A photo detecting unit, which achieves the fifth object of the present invention, receives at the photo detecting elements the light beams of at least 3 by 3 in the form of a parallelogram, the received reflecting light beams being formed in a manner that a light beam is split into at least three light beams of the 0-th order and the xc2x11-st order of diffraction by a diffraction grating, the split light beams are each split into at least three light beams by a Wollaston prism, the photo detecting element includes a first photo detecting element for receiving the light beams of the xc2x11st order of diffraction, a second photo detecting element for receiving the light beams of the xe2x88x921st order of diffraction, a third photo detecting element, consisting of a plural number of photo detecting elements, for receiving in divided form the central light beam of the three light beams of the 0-th order of diffraction, and a pair of fourth photo detecting elements for receiving respectively the reflecting light beams located on both sides of the central reflecting beam of the three light beams split by the Wollaston prism of the 0-th order, an angle between a line connecting the centers of the first and second photo detecting elements and a line between the paired fourth photo detecting elements are not a right angle.
According to the sixth aspect of the invention, the magneto-optical recording/reproducing pickup device comprises: a light source for generating light beams; a diffraction grating for splitting the light beam emitted from the light source into at least three light beams of the xc2x11st order of diffraction and the light beam of the 0-th order of diffraction; an objective lens for converging the light beams split by the diffraction grating on the recovery surface of a magneto-optical recording medium, and receiving the reflecting light beams from the recording medium, a Wollaston prism for splitting each of the reflecting light beams coming in through the beam splitter into at least three light beams, the direction of the splitting by the Wollaston prism being at an angle to the direction of the splitting by the diffraction grating; a photo detecting unit for receiving at least nine reflecting light beams from the Wollaston prism, the photo detecting unit including a first photo detecting element for receiving the light beams of the +1st order of diffraction, a second photo detecting element for receiving the light beams of the xe2x88x921st order of diffraction, a third photo detecting element, consisting of a plural number of photo detecting elements, for receiving in divided form the central light beam of the three light beams of the 0-th order of diffraction, and a pair of fourth photo detecting elements for receiving respectively the reflecting light beams located on both sides of the central reflecting beam of the three light beams split by the Wollaston prism of the 0-th order, an angle between a line connecting the centers of the first and second photo detecting elements and a line between the paired fourth photo detecting elements being not a right angle; and lens drive means for driving the objective lens for positioning adjustment.
In the photo detecting unit, an angle between a line connecting the centers of the first and second photo detecting elements and a line between the paired fourth photo detecting elements is not a right angle. Accordingly, the photo detecting element is well adaptable for an optical system in which the direction of the splitting by the diffraction grating using the plate beam splitter capable of causing an astigmatism is at an angle, not a right angle, to the direction of the splitting by the Wollaston prism.